Method for shot peening

ABSTRACT

The present invention is to provide a method for shot peening for producing a compressive residual stress that exceeds 60% of the yield strength at 0.2% offset without using stress shot peening. Shot media are peened onto a processed steel that has an amount of retained austenite in a range between 5 to 30%, and any change in the amount of retained austenite is controlled to be in a range of 2 to 30% before and after shot peening to produce the compressive residual stress in the processed steel.

TECHNICAL FIELD

The present invention relates to a method for shot peening.Specifically, it relates to a method for shot-peening a steel.

BACKGROUND ART

Conventionally, shot peening has been known to produce compressiveresidual stresses to improve the fatigue strength of parts made of asteel (see authored by the Society of Shot Peening Technology of Japan;Fatigue of Metals and Shot Peening; published by Gendai Kogaku-sha;2004). Further, it has been known that increasing the maximum value ofcompressive residual stresses is very effective in improving the fatiguestrength of the parts (see Masahiko Mitsubayashi, Takashi Miyata, andHideo Aihara; Prediction of Improvement in Fatigue Strength by ShotPeening and Selection of Most Effective Peening Conditions; Transactionsof JSME, Vol. 61, No. 586 (June, 1995) pp. 28-34).

However, it is also known that the maximum value of compressive residualstresses produced by shot peening is approximately 60% of the yieldstrength at 0.2% offset (Hideki Okada, Akira Tange, and Kotoji Ando;Relationship among Specimen's Hardness, Residual Stress Distribution andYield Stress on the Difference of Shot Peening Methods; Journal of HighPressure Institute of Japan, Vol. 41, No. 5 (2003) pp. 233-242). Thus byapplying stress shot peening, i.e., shot-peening a part that is under apre-stressed condition, a maximum compressive residual stress thatexceeds 60% of the yield strength at 0.2% offset can be obtained (seethe above reference).

Though the stress shot peening can be used for a part, like a springthat can be stressed while shot-peening it, there have been problems inthat stress shot peening cannot be used for a part like a gear thatcannot be stressed while shot-peening it.

DISCLOSURE OF THE INVENTION

The object of the present invention is to provide a method for shotpeening for producing maximum compressive residual stresses that exceed60% of the yield strength at 0.2% offset by controlling the propertiesof the material or the conditions for the heat treatment of theprocessed steel and the conditions for shot peening, without using thestress shot peening.

The method for shot peening of the first aspect of the present inventionis to produce a compressive residual stress in a processed steel thathas an amount of retained austenite in a range of 5 to 30%, by peeningshot media onto the processed steel. The amount of retained austenite iscontrolled to keep the change in the amount within a range of 2 to 30%before and after the shot peening.

In the method for shot peening of the second aspect of the presentinvention, the shot peening is controlled to keep the change in theamount of retained austenite at the depth where the maximum compressiveresidual stress is generated at a range of 2 to 30% before and after theshot peening.

In the method for shot peening of the third aspect of the presentinvention the processed steel is a gas carburized steel.

By the method for shot peening of the first aspect, a maximumcompressive residual stress can be obtained that exceeds 60% of theyield strength at 0.2% offset. Thus no jig for stressing the processedsteel for the shot peening is required. Further, efficient shot peeningcan be used for a part such as a gear that has a complicated shape.

By the method for shot peening of the second aspect, the method for shotpeening of the first aspect can always be performed.

By the method for shot peening of the third aspect, a processed steelthat has a desired amount of retained austenite can be easily obtainedby changing carburizing.

The basic Japanese patent application, No. 2010-176682, filed Aug. 5,2010, is hereby incorporated by reference in its entirety in the presentapplication.

The present invention will become more fully understood from thedetailed description given below. However, the detailed description andthe specific embodiment are illustrations of desired embodiments of thepresent invention, and are described only for an explanation. Variouspossible changes and modifications will be apparent to those of ordinaryskill in the art on the basis of the detailed description.

The applicant has no intention to dedicate to the public any disclosedembodiment. Among the disclosed changes and modifications, those whichmay not literally fall within the scope of the present claimsconstitute, therefore, a part of the present invention in the sense ofthe doctrine of equivalents.

The use of the articles “a,” “an,” and “the” and similar referents inthe specification and claims are to be construed to cover both thesingular and the plural, unless otherwise indicated herein or clearlycontradicted by the context. The use of any and all examples, orexemplary language (e.g., “such as”) provided herein, is intended merelyto better illuminate the invention, and so does not limit the scope ofthe invention, unless otherwise claimed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a table showing the properties of the processed steels thatwere used in the embodiments of the present invention. FIG. 2 is a tableshowing the conditions of the shot peening that were used in theembodiments of the present invention.

FIG. 3 is a table showing the properties of the processed steels afterthe shot peening.

FIG. 4 is a supplemental table giving data that are similar to those inTable 3.

BEST MODE FOR CARRYING OUT THE INVENTION

Below, the embodiments of the present invention are described withreference to the drawings.

FIG. 1 is a table showing the properties of the processed steels thatwere used in the embodiments of the present invention. Steel-A toSteel-G are prepared as the processed steels. The carbon contents (wt%), the conditions for heat treatment, and the yield strengths at 0.2%offset (MPa), as properties of the materials, and the tensile strengths(MPa), the hardness at the surfaces (HV0.3), and the amount of retainedaustenite γ (Gamma)_(R) (%),

are all shown in the table. The processed steels are prepared from thesteels that are based on a chromium steel or a chromium-molybdenum steeland that have different carbon contents, i.e., between 0.2 and 0.8 wt %,and the steels that are based on a chromium-molybdenum steel that have acarbon content of 0.8 wt %, and that are tempered in differentconditions. These processed steels are gas carburized steels.

FIG. 2 is a table showing the conditions of the shot peening that wereused in the embodiments of the present invention. Two types ofconditions for shot peening (the conditions for peening shot media ontothe processed steels) were used. A compressive-air shot peening systemwas used in both types. The hardness (HV), the diameters (mm), and theair pressure for peening shot media are all shown in the table. Thecoverage, which represents the amount of shot media being peened, was300% in all cases.

FIG. 3 is a table showing the properties of the processed steels afterthe shot peening. The table also shows the properties before the shotpeening. It shows the properties of Steel-A to Steel-G in the upper andlower sides for two respective types of conditions for shot peening.

That table shows the maximum compressive residual stress

σ (Sigma)_(R) (MPa),

Gamma_(R) at the peak depth (%), Gamma_(R) (max)/Gamma_(0.2), and therate of change in Gamma_(R) at the peak depth (%), as the properties ofthe processed steels after shot peening.

The maximum compressive residual stress Gamma_(R) (MPa) means themaximum value of the compressive residual stresses that are measured atvarious depths from the surface (since a compressive residual stress isgenerally expressed as a negative value, it is the maximum value inabsolute values). The compressive residual stresses were measured byusing a micro-stress analyzer that is available from Rigaku Corporation(X-ray tube: Cr-Kα(_(Alpha)); diffractive surface: (220); stressconstant: −3] MPa/deg; Bragg angle of the strain-free 2θ: 156.4°).

The Gamma_(R) at the peak depth (%) denotes the amount of retainedaustenite at the depth where the maximum compressive residual stress isgenerated. The amounts of retained austenite were also measured by usinga micro-stress analyzer that is available from Rigaku Corporation (X-raytube: Cr—K_(Alpha); diffractive surface: (220); Gamma-diffraction plane:(311); time for measuring on Alpha-plane: 60 sec; range of diffractionon Alpha-plane: 156.4 degree C.).

The Gamma_(R) (max)/Gamma_(0.2) denotes the maximum compressive residualstress compared to the yield strength at 0.2% offset. The rate of changein Gamma_(R) at the peak depth (%) denotes a rate of change in theamount of retained austenite before and after the shot peening at thedepth where the maximum compressive residual stress is generated.

As seen in FIG. 3, the Gamma_(R) (max)/Gamma_(0.2) exceeds 60%, which isthe target value, for Steel-B, -C, -D, -E, and -G. FIG. 4 showssupplemental data for FIG. 3.

From these data, it was found that the processed steels that have themaximum compressive residual stress that exceeds 60% of yield strengthat 0.2% offset can be obtained by the following process, i.e., peeningshot media onto a processed steel that has the amount of retainedaustenite in a range between 5 to 30%. The rate of change (reduction) inthe amount of retained austenite at the depth where the maximumcompressive residual stress is generated is controlled to be in a rangebetween 2 to 30%.

The threshold value of the amount of retained austenite, i.e., 5 to 30%,is determined based on the maximum value in the range that isrepresentative for industrial materials. The upper limit for the rate ofchange in the amount of retained austenite, i.e., 30%, is specifiedbased on the maximum value of the amount of retained austenite. Thelower limit for the rate of change in the amount of retained austenite,i.e., 2%, is determined by plotting the Gamma_(R) (max)/Gamma_(0.2) inrelation to the rate of change in Gamma_(R) at the peak depth (%) anddrawing an approximate curve by the least square method.

If the rate of change (reduction) in the amount of retained austenite ofthe processed steel at the depth where the maximum compressive residualstress is generated is controlled to be in a range between 2 to 30%, themaximum compressive residual stress becomes over 60% of the yieldstrength at 0.2% offset. This is because the retained austenite expandsby the deformation-induced martensitic transformation and thus themechanical properties improve by the expansion of the retainedaustenite.

As discussed above, in the embodiments of the present inventionprocessed steels that have the amount of retained austenite in a rangebetween 5 to 30% are subject to shot peening. The change in the amountof retained austenite before and after shot peening is controlled to bein a range of 2 to 30%, so as to produce the compressive residual stressin the processed steel. Thus, a maximum compressive residual stress thatexceeds 60% of the yield strength at 0.2% offset can be produced.Therefore, no jig for stressing the processed steel for the stress shotpeening is required. Further, a part such as a gear, which has acomplicated shape, can be efficiently shot-peened.

Further, by changing the amount of retained austenite at the depth wherethe maximum compressive residual stress is in the range between 2 to 30%before and after shot peening, a maximum compressive residual stressthat exceeds 60% of the yield strength at 0.2% offset can always beproduced.

Further, since the processed material is a gas carburized steel, aprocessed steel that has a desired amount of retained austenite can beeasily obtained by adjusting the conditions for carburizing.

Any steels can be used for the processed steels, but a gas carburizedsteel that has a large amount of retained austenite is preferable.

1. A method for shot peening, wherein shot media are peened onto aprocessed steel that has an amount of retained austenite in a rangebetween 5 to 30%, and wherein a change in the amount of retainedaustenite is controlled to be in a range of 2 to 30% before and aftershot peening to produce a compressive residual stress in the processedsteel.
 2. The method for shot peening of claim 1, wherein a change inthe amount of retained austenite at the depth where the maximumcompressive residual stress is generated is controlled to be in a rangeof 2 to 30% before and after shot peening to produce a compressiveresidual stress in the processed steel.
 3. The method for shot peeningof claim 1 or 2, wherein the processed steel is a gas carburized steel.